The present invention relates to colour matrix display units that provide an image in the form of elementary points or "pixels", distributed in a regular array. It relates more particularly to matrix display units of the type including at least three sets of pixels that are respectively green, red, and blue, with the pixels in each set being distributed in a matrix comprising rows and columns, all of the pixels in the same row being simultaneously addressed for writing by respective column conductors, the total active or light-emissive area of the pixels frequently being the same for the three sets so as to achieve a colour balance.
The invention is applicable to very many types of matrix display units, and particular mention may be made of liquid crystal screens (regardless of whether they are of the multiplexed type or of the active matrix type), electrically controlled light emitting screens, screens having an array of light emitting diodes (LEDs), and plasma screens.
In conventional colour matrix display units, each coloured pixel (which for convenience is referred to below as a "macropixel") is made up by juxtaposing at least three pixels each allocated to one of the primary colours (usually green, red, and blue). The brightness of each pixel is modulated, either by adjusting a duty ratio, or more commonly by adjusting luminance, so as to obtain the desired colour.
Several pixel dispositions are widely used. Mention may be made of arrangements in strips (where the three pixels making up the same macropixel are disposed side by side along a common straight line), in triplets (with the three pixels being disposed at the apexes or corners of an equilateral triangle), and in quads (where four pixels are disposed at the corners of a square and represent the three primary colours together with an additional colour or white).
A major problem in making matrix display units lies in the density of the connections to be made. The number of connections between the display proper and its controlling electronics is equal to the number of rows plus the number of columns: for a screen where each macropixel comprises three pixels in a strip, 2 080 connections are required for 640 rows of 480 macropixels. The failure of any connection will cause one or more pixels to go out and will degrade the image. Unfortunately, the risk of failure increases with routing complexity.
Making each macropixel from three (or more) pixels also has the drawback of reducing the aperture factor, i.e. the active fraction of the area of the display unit, or that fraction which actually emits light. Each pixel must be separated from its neighbours by an opaque line in order to allow for manufacturing tolerances and to avoid interference between adjacent pixels. For example, the aperture factor of a present day active matrix liquid crystal screen hardly exceeds 50%. Unfortunately, any reduction in aperture factor can be compensated only by more powerful back lighting.